1. Field of the Invention
The present invention relates to a crankcase structure of an engine mounted on a vehicle such as a motorcycle.
2. Description of the Related Art
In an internal combustion engine such as a gasoline engine, explosion energy generated in an upper part of a piston is transmitted to a crankshaft via a connecting rod, and thereby the crankshaft is rotated. The crankshaft is supported by a crankcase via a bearing, and quite a large load due to engine explosion acts not only on the bearing but also on a bearing supporting portion of the crankcase. Since the crankshaft rotates at a high speed while receiving such a large load, high rigid strength is required of a shaft receiving bearing which supports the crankshaft and a vicinity of the bearing supporting portion.
Here, the above will be explained by using a concrete example of an engine of this kind. For example, in a four-cycle single-cylinder engine 10 shown in FIG. 8 and FIG. 9, a crankcase 11 housing and supporting a crankshaft 12 in a rotatable manner, a cylinder 15 housing, in a vertically movable manner, a piston 14 coupled to the crankshaft 12 via a connecting rod 13, a cylinder head 17 housing a valve driving device 16, and a cylinder head cover 18 attached to cover the cylinder head 17 are connected in series in an approximately vertical direction.
As shown in FIG. 9, an intake port 19 and an exhaust port 20 which are each communicated with a combustion chamber 21 are formed in the cylinder head 17. The intake port 19 and the exhaust port 20 are opened and closed at a predetermined timing by an intake valve 22 and an exhaust valve 23 driven by the valve driving device 16.
In this example, as shown in FIG. 8, the crankcase 11 is configured to be right-and-left two-split along a split surface S. In this case, as shown in FIG. 10, a crank chamber 24 is formed by a right side crankcase 11R and a left side crankcase 11L, and in the crank chamber 24 a crankshaft 12 and a right and left pair of crank webs 12A which rotates integrally with the crankshaft 12 are axially supported in a manner to rotate freely. The connection rod 13 is coupled between each crank webs 12A, via a crank pin 25. Note that in a surrounding of the crank chamber 24, a mission chamber 26 is disposed in a rear side behind a case partition, a clutch chamber 27 is disposed in a right side thereof, and a magneto chamber 28 is disposed in a left side, adjacently to each other.
The crankshaft 12 is supported by a pair of bearings 29. In this example, as the bearing 29, a ball roller bearing is used, but a cylindrical roller bearing or a metal slide bearing can be used. The respective bearings 29 are fit into bearing holding portions 30 provided in the right side crankcase 11R and the left side crankcase 11L respectively and are fixedly supported inside the bearing holding portions 30. The bearing holding portion 30 has a cross section of an approximately band shape as indicated by a dashed line in FIG. 10, in order to equalize a fastening margin to the bearing 29, that is, a press-fit margin or a deformation margin.
At a time of engine explosion, as shown in FIG. 9, its explosion pressure P is received by a piston 14, and transmitted from the piston 14 to the crankshaft 12 via the connecting rod 13. The crankshaft 12 rotationally moves, and its explosive force simultaneously acts from the crankshaft 12 to the bearing holding portion 30 as a load F1 as indicated in FIG. 11. The explosive pressure P also acts on a cylinder head 17 side. Here, as shown in FIG. 8, the cylinder head 17 is fastened to the crankcase 11 by cylinder head bolts 31. In this case, the cylinder head bolts 31 are screwed to female screws 33 formed in a boss portion 32 provided in the crankcase 11. The explosive pressure P acting on the cylinder head 17 side acts on the crankcase 11 via the cylinder head bolt 31 as a load F2.
Patent Document 1: Japanese Laid-open Patent Publication No. 2009-243440
Action of the load F1 and the load F2 as above at the time of engine explosion causes occurrence of a stress-strain E1 in a cylinder axis line direction and a stress-strain E2 in an orthogonal direction to the cylinder axis line in the cylinder case 11, if no measure is taken. In order to suppress occurrence of a crack or a deformation of the crankcase 11 due to the stress-strains E1, E2, ribs 34 are provided between the bearing holding portion 30 and the boss portions 32. Though rigidity can be strengthened by providing the ribs 34, it is not always sufficient. That is, for the bearing holding portion 30 and the ribs 34, it is difficult to completely suppress a deformation responding to the stress-strains E1, E2 as indicated by two-dot chain lines of FIG. 11.
Note that Patent Document 1 discloses an example of a rib of this kind.